5.0 Traffic Flow Improvements

Traffic flow improvement programs that achieve emission reductions.
Section 108 (v), CAAA

Introduction

Traffic flow improvements include a very wide range of measures for improving the operational efficiency of an intersection or corridor, generating small increases in capacity or delay reduction without the addition of extra lanes or new roads. The logic behind this emission reduction strategy is that reducing congestion and delays will also decrease congestion-related emissions. Traffic flow improvements have been used for decades, with projects becoming increasingly more complex as congestion on U.S. roadways has worsened.

Improvements generally provide a cost-effective method to reduce congestion although their effects on vehicular traffic can be difficult to quantify. Also, once traffic is less congested and flows more efficiently, motorists may increase vehicle trips, leading to increased VMT and increased emissions. Planners should be aware of the difficulties in quantification of the benefits of the strategy because of the potential increases in VMT.

Main Components

Strategies to improve traffic flow can be grouped into four general types:

  • Traffic signalization,

  • Traffic operations,

  • Enforcement and management, and

  • Intelligent Transportation Systems (ITS).

Traffic signalization represents the most common traffic management technique applied in the United States. Traffic signal improvements can include the following:

  • Updating traffic signal hardware to utilize more modern technology, allowing for more sophisticated traffic flow strategies to be planned;

  • Timing traffic signals to correspond with current traffic flows, reducing unnecessary delays;

  • Coordinating and interconnecting signals to better interface pre-timed and traffic actuated signals, actively managed timing plans, and master controllers to minimize the number and frequency of stops necessary at intersections; and

  • Removing signals at intersections no longer requiring signalized stop control to reduce vehicle delays and unwarranted stops on the major street.

Traffic operations describe several types of roadway improvement projects, including:

  • Converting two-way streets to one-way operation to improve corridor travel times and increase roadway capacity;

  • Restricting left turns on two-way streets as a means of eliminating conflicts with left-turn movements, thereby reducing congestion and delay;

  • Separating turning vehicles from through traffic with continuous median strip turn lanes;

  • “Channelizing” roadways and intersections (i.e., clearly marking travel lanes and paths with striping and signage to reduce motorist confusion and uncertainty by channeling traffic into the proper position on the street) to improve vehicular flow and capacity; and

  • Widening and reconstructing short sections of roadways and intersections to reduce bottlenecks along sections where traffic capacity is below that of the adjacent street (e.g., traffic islands, roundabouts, turning lanes, and signage).

Several types of programs fall under enforcement and management:

  • Incident management systems consist of roving tow or service vehicles, motorist aid call boxes, incident teams, signage systems, contingency planning, and improved information availability to consumers through radio and television.

  • Ramp metering, a technique for improving traffic flow on freeways, uses signals to regulate traffic entering the highway to pre-timed intervals or to intervals determined by traffic volumes on the ramp or the highway.

  • Enforcement of traffic and parking program regulations necessary for individuals to adapt or adhere to particular travel and parking behaviors.

ITS applies information processing, communications technology, advanced control strategies, and electronics to improve the safety and efficiency of a transportation system. In the context of mobile source emission reduction strategies, ITS emphasizes advanced traffic control, incident management, and corridor management. This area includes the following:

  • Transportation management centers (TMCs) contain closed-circuit monitors for observing traffic conditions. Cameras are placed along sections of freeways or arterials commonly congested during commute hours. These cameras enable TMC personnel to observe traffic and respond to situations in a timely manner, reducing adverse effects on the commuting traffic. TMCs serve as information and communication conduits between transportation personnel and law enforcement officials.

  • The Congestion Management System (CMS), a decision support tool, provides an integrated approach to planning by assessing information on all asset inventories, including condition and operational performance. Designed to assist decision makers in choosing cost-effective strategies and actions, CMS is a systematic approach to improving the efficiency of transportation assets. CMS is a tool for data management, analysis, and deficiency identification for all state highway assets, as well as local roadways. CMS uses historic, current, and forecasted attributes to help identify current and future congested roadways. It also incorporates travel demand forecasting capabilities for urban and rural areas to assess transportation system performance, identifying areas where it is unacceptable. Performance measures with localized thresholds allow CMS to address movement of people, vehicles, and goods based on goals and objectives in specific areas.

Other Considerations

Typically, city and county public works departments implement traffic flow improvements with financial assistance provided by state and federal funding sources. Because these actions facilitate urban driving, there is usually little public opposition, except perhaps for local residents who may object to disruptions caused by construction.

Many small jurisdictions and even some large central cities have limited traffic engineering capabilities and budgets. In those cases, traffic signal management and roadway maintenance and design are often limited to the most basic or rudimentary installation and maintenance functions.

Implementing programs of interrelated traffic flow enhancement strategies can lead to substantial reductions in travel time and delay. Combined with signalization improvements and enforcement, traffic operations can fundamentally affect circulation in a relatively large area, improving system travel speed and efficiency overall. For any improvement to be successful, good coordination must exist between state and local traffic agencies and the police department assigned enforcement responsibilities.